1. Field of the Invention
The present invention relates to a backlight module and circuit structure thereof. More particularly, the present invention relates to a multi-tube backlight module and feedback circuit structure thereof.
2. Description of Related Art
With the rapid development of electronic industry, electronic products including mobile phones, digital cameras, digital camcorders, notebook computers, desktop computers are striving towards easier operation, more powerful functions and a better appearance. However, all these mobile phones, digital cameras, digital camcorders, notebook computers and desktop computers demand a suitable display screen to serve as a man-machine interface and facilitate various operations. In recent years, liquid crystal display (LCD) panels have become the mainstream display screens for most of the mobile phones, digital cameras, digital camcorders, notebook computers and desktop computers. Because a LCD panel will not emit light on its own, a backlight module must be disposed underneath the display panel to provide a planar light source having a sufficient brightness and contrast for producing a clear image on the screen.
In general, a backlight module can be classified into a side-edge type and a direct-type backlight module. In a side-edge type backlight module, a linear light source is disposed on one side of a light-guiding plate so that light entering the light-guiding plate is transformed into a planar light source for the liquid crystal panel. In the direct type backlight module, light sources are disposed under the liquid crystal panel so that the light sources are able to provide a planar light source directly. Thus, if the demanded brightness level for a liquid crystal panel is high, a direct type backlight module is often deployed. At present, there are two major methods for constructing the planar light source of a direct type backlight module. One type of backlight module utilizes an array of light-emitting diodes to form the planar light source while another type of backlight module utilize a plurality of parallel-aligned cold cathode fluorescent lamps (CCFL) (also referred to as a multi-tube backlight module in the following).
The cold cathode fluorescent lamps (CCFL) of a conventional multi-tube backlight module are driven by an alternating current power source. The powering device connects all the power terminals of the CCFL to provide a driving current to all the CCFL. To gauge the actual current passing through the tube and maintain a constant light emission, the low voltage terminal (the feedback terminal) of each CCFL is equipped with a feedback circuit for transmitting a feedback signal to a feedback control device (for example, a pulse width modulation (PWM) controller). Through the feedback signal from the tube, the feedback control device senses the current flowing inside the tube and controls the power source in real time to provide a compensating feedback current so that the CCFL can maintain a constant output.
FIG. 1 is schematic circuit diagram of a conventional multi-tube backlight module. As shown in FIG. 1, the multi-tube backlight module 100 comprises a plurality of cold cathode fluorescent lamps (CCFL), a driving module 120 and a plurality of feedback lines 130. The driving module 120 further comprises a powering device 122 and a feedback control device 124. The powering device 122 is coupled to a power terminal 112 of each lamp 110. The power device 122 is capable of converting a direct current power source into an alternating power source for driving the CCFL 110. Furthermore, a feedback terminal (low voltage terminal) 114 of each CCFL is coupled to the feedback control device 124 through the feedback line 130 for transmitting a feedback signal to the feedback control chip 124. After receiving the feedback signal, the feedback control device 124 provides a feedback compensation of the driving current so that a constant output from the CCFL 110 is maintained.
It should be noted that the aforementioned multi-tube backlight module has an independent feedback mechanism. In other words, each cold cathode fluorescent tube has a feedback line for transmitting a feedback signal to the driving module. One major advantage is that the current in each CCFL can be accurately detected so that precise feedback compensation is possible. On the other hand, more feedback lines and connecting components are required. Hence, the circuit is more complicated and costly to fabricate.
FIG. 2 is a schematic circuit diagram of another conventional multi-tube backlight module. As shown in FIG. 2, the multi-tube backlight module 200 uses common ground feedback to control the cold cathode fluorescent tube 210. The cold cathode fluorescent tubes 210 are divided into two groups. The CCFL 210 in each group utilizes a single feedback line 230 to connect with a feedback control device 224 inside a driving module 220 so that the feedback control device 224 is capable of compensating the driving current to each group of CCFL 210. Although the common ground feedback method is able to lower the production cost and simplify the feedback circuits, the detected feedback current is that of a group of CCFL and hence precise control of each CCFL is impossible. Furthermore, interference between neighboring cold cathode fluorescent tubes is also possible.
In other words, the two aforementioned feedback systems have its advantages as well as its disadvantages. In general, the type of backlight module chosen depends on cost and quality requirements. However, because the two feedback types have different feedback terminals and circuit layouts, a different circuit board is required. Since the same feedback circuit cannot be used in backlight module having a different feedback system, a new circuit has to be designed and developed.